been observed with water droplets on numerous occasions. Wolfhard and Parker (1.2) showed that small organic droplets (diameters less than 20 microns) evaporated with sufficient rapidity so that the fine mist or vapor which resulted possessed essentially the combustion characteristics of a completely vaporized fuel. For the three solvents, droplets with a diameter less than 20 microns constituted these nuniber percentages: 4-methyl pentan-2-one, 89%; chloroform, 78%; and water, 69%. Since the amount of atomic and molecular excitation in the flame is dependent upon the number of droplets reaching the reaction zone and completely vaporizing before entering that energetic region, i t would be expected that an aerosol which contained a greater number of small diameter
droplets will provide a higher number of excitable species in the flame. This has been borne out in practice and has given rise to the widespread use of organic solvents, particularly 4-methyl pentan-a-one, in flame spectrophotometry. LITERATURE CITED
(1) Green, H. L., Lane, IT. R., “Partic-
ulate Clouds: Dusts, Smokes and Mists,” Van Nostrand, London, 1957. (2) Gudkov-Belyakov, V. K., Dizelestroenie 7, 10 (1940); C.A. 37, 4548 (1943). (3) Lewis, B., Pease, R . S . , Taylor, H. S., “Combustion Processes,” F’ol. 11, Princeton UniverFity Preps, Princeton, N. J., 1956. (4) Mavrodinennu, R . , Boiteux, H., “L’Analyse Spectrale Quantitative par la Flamme,” Masson et Cie, Paris, 1954.
(5) May, K. R., J. ,Sa. Instr. 2 2 , 187 (1945). (6) lbid., 27, 128 (1950). (7) Mugele, R. A., Evans, H. D., Ind. Eng.Chem.43,1317(1951). (8) Nukiyama, S., Tanasawa, Y., T r a n s . SOC. Mech. Engrs. ( J a p a n ) 5 , (18) 68 (1939). (9) Pilcher, J. &I., U. S. Air Force WADC TR 56-344,Chap. 4, Section I, (March 1957). (10) Probert, R. P., Phil. M u g . 37, 94 ( 1946). (11) Smit, J. A., Alkemade, C. Th. J., Verschure, J. C. M., Biochim. et Biaphys. Acta 6, 508 (1951). (12) Wolfhard, H. G., Parker, W. G., J.Inst. Petrol. 35, 118 (1949). RECEIVEDfor review March 20, 1961. Accepted Piovember 30, 1961. Taken from a portion of a dissertation submitted by William J. Carnes t o the Graduate School of The University of Tennessee in partial fullfilment of the requirements for the degree of Doctor of Philosophy, December 1961. Eastern Analytical Symposium, Xew York, November 1960.
Determination of Hydroxyl Number of Polyoxyalkylene Ethers by Acid -Cata Iyzed Acetylation R. S. STETZLER and C. F. SMULLIN Atlas Chemical Industries, Inc., Wilmington, Del. ,An acid-catalyzed acetylation method applicable to the determination of hydroxyl values of polyethers is described. p-Toluenesulfonic acid is used as the catalyst and acetic anhydride in ethyl acetate as the acetylating reagent. The method is rapid and requires no special apparatus. Precision and accuracy are better than obtained by previous acetylation methods.
P
oxide adducts of sorbitol have recently gained widespread use in the manufacture of rigid urethane foams. The physical properties of these foams vary widely, depending upon the ratio of h ~ d r o u y lto isocyanate. I t is therefore important to have a rapid, simple, and reliable analytical method to determine the hydroxyl content of thew polyethers. The methods most commonly used for the determination of hydroxyl groups involve acetylation using acetic anhydride in pyridine, phthalation using phthalic anhydride in pyridine, and determination of actire hydrogen using Grignard reagent. An excellent review of the pertinent literature on these methods through 1952 is given by Mehlenbacher (4). Burns and Nuraca (1) used infrared spectroscopy to determine the hydroxyl concentration in polypropylene glycols. The use of perOLYPROPYLENE
194
ANALYTICAL CHEMISTRY
chloric acid to catalyze the acetylation of organic hydroxyl groups with acetic anhydride in ethyl acetate has been reported by Fritz and Schenk ( 3 ) . The same authors (3) mention briefly the possible use of p-toluenesulfonic acid, particularly for reactions carried out at elevated temperatures. Recently, Siggia, Hanna, and Culmo ( 5 ) employed pyromellitic dianhydride as the esterifying reagent, using tetrahydrofuran as the solvent. Base-catalyzed acetylation and phthalation reactions are slow, particularly when applied to secondary hydroxyl groups as found in polypropylene glycols and other propylene oxide adducts. Burns (1) found that an acetylation time of about 4 hours is required to acetylate polypropylene glycols quantitatively. The infrared method and the active hydrogen method both have the disadvantage that water interferes in the determination. Therefore, for precise results the sample must be anhydrous or the water content must be measured by a separate technique, usually the Karl Fischer procedure, and the final results corrected. Esterification methods employing perchloric acid as the catalyst are not applicable to polyethers, because of the attack of perchloric acid upon the ether groups ( 2 ) . This was confirmed in this laboratory.
With the present method, the hydroxyl content of polyethers is determined by acetylation in ethyl acetate solution, using p-toluenesulfonic acid to catalyze the reaction. Complete acetylation is obtained in 15 minutes a t 50’ C. The amount of hydroxyl is calculated from the difference between the blank and sample titrations with alcoholic potassium hydroxide. REAGENTS AND SOLUTIONS
2 M Acetic Anhydride in Ethyl Acetate. Add 14.4 grams of reagent grade p-toluenesulfonic acid (CH3C e H 4 S 0 3 H . H z 0 )to 360 ml. of ACS grade ethyl acetate in a clean, dry 500-ml. amber glass reagent bottle. Agitate the mixture with a magnetic stirrer until t h e acid is completely dissolved. Add, slowly, 120 ml. of ACS grade acetic anhydride, maintaining agitation during the addition. A slight yellow color will develop, but the anhydride content of the reagent will remain a t a satisfactory level for several days. 0.56N Potassium Hydroxide. Dissolve 334 grams of reagent grade potassium hydroxide in 2000 ml. of absolute methanol. Cool and filter t o remove precipitated carbonates. Dilute t h e filtrate t o 9000 ml. with absolute methanol. Standardize against primary standard potassium acid phthalate.
Mixed Indicator. Mix 1 p a r t of 0.1% neutralized cresol red with 3 parts of 0.1% neutralized thymol blue. Potassium Acid Phthalate, primary standard grade. PROCEDURE
Weigh accurately a sample containing 5 to 6 nimoles of hydroxyl into a 250-nil. Erlenmeyer flask. Pipet 5 nil. of 2.U acetic anhydride in ethyl acetate into the flask. Cover the flask mith a natch glaes and immerse to just above the liquid level in a constant temperature water bath maintained a t 50" f 1' C. After 5 minutes, remove the flask from the bath and swirl gently. Return the flask to the bath and allow the reaction to proceed for a n additional 10 minutes. Add 2 ml. of distilled water and swirl the mixture vigorously. Add 10 ml. of 3 to 1 pyridine-water solution, rinsing the sides of the flask during the addition. 24110w the flask to stand 5 minutes a t room temperature to hydrolyze the excess acetic anhydride completely. Titrate with 0.56W potassium hydroside using the mixed indicator. Take the color change from yellow to blue-violet as the end point. R u n a reagent blank in the manner described above, omitting only the sample. Use the difference between the blank, Vb, and the sample, Vs, to calculate the hydroxyl content of the sample. Determine any free acidic or basic materials present in the sample on a separate sample, and correct the final analysis accordingly. DISCUSSION AND RESULTS
The samples used in this study were polypropylene oxide adducts of sorbitol. The hydroxyl content of these products is dependent upon the ratio of propylene oxide to sorbitol. The catalytic effect of several acids on the acetylation is given in Table I. Acetylating reagents iwre prepared by mixing 3 volumes of ethyl acetate with 1 volume of acetic anhydride and sufficient acid t o make the final solution 0.15 to 0.20.U with respect to the particular acid used. Perchloric acid attacks the ether groups in the polyosypropylene chain even a t room temperature to yield apparent hydroxyl values which are 20 t o 30Y0higher than the theoretical. The attack of perchloric acid on the ethers produces a brown color similar t o t h a t reported by Fritz ( 3 ) when attempting to acetylate tetrahydrofurfuryl alcohol, where perchloric acid causes ring opening. Reacxtions employing hydrochloric acid as the catalyst are slow, yielding only S570 reaction after 120 minutes a t 50' C. The proposed method, employing p toluenrsulfonic acid as the catalyst, produces quantitative acetylation in 10 to 15 minutes a t 50' C. without any
evidence of attack on the ether groups after 60 minutes a t this temperature. Table I1 shows the precision and accuracy of the proposed method when applied to the acetylation of various polypropylene oxide adducts of sorbitol. Similar data obtained using the conventional pyridine-catalyzed acetylation procedure are shown for comparison. Both precision and accuracy are improved significantly n ith a substantial reduction of reaction time. The scope of this method is not limited t o polypropylene oxide adducts of sorbitol. Fatty acid esters, alkylated phenols, fatty alcohols, and polyoxyethylated compounds mere quantitatively acetylated, as shown in Table 111. While this method is not as rapid as the perchloric acid-catalyzed methods (S), it is more rapid than conventional base-catalyzed acetylation procedures and suffers no interference from
Table 11.
Table !. Comparison of Acid Catalysts on Acetylation of Sorbitol Plus 10 Moles of Propylene Oxide Derivative
Reaction Reaction c, Temp., Time, /G Acid C. Min. Reaction" Perchloric R.T.b 5 117 1I.T. 10 120 15 128 R.T. 1i.T. 30 129 O
Hydrochloric
R.T. 50 50
30 60 90 120
50 50 50
10
50 p-Toluenesulfonic
50
10 67 i(i
85 98 101
0
2.0
101
60
101
Per cent of theoretical calculated value, from charge weights. b Room temperature. 0
Precision and Accuracy of Method
Hydroxyl Number Calculate& Foundb-
Derivative Sorbitol f 6 moles propylene oxide +10 moles propylene oxide
661 49 1
++14 moles propylene oxide 10 moles propylene oxidec
392 49 1
658 =k 4 . 3 Prep. 1 494 =k 3 . 2 Pren. 2 492 =k 4 . 3 Pie;. 3 500 =k 4.0 395 i 1 . 8 Prep. 3 486 f 7 . 7
01 /0
Accuracy 99.6 f 0 . 6 5 100.6 =k 0 . 6 5 100.2 f 0.88 101.8 i 0.81 100.8 f 0.46 99.0 f 1 . 6
a hlilligrams of potassium hydroxide equivalent t o hydroxyl content of 1-grain sample, calculated from actual charge weights of ingredients. Average of 25 to 40 determinations using 15-minute reaction at 50' C. c Comparable data obtained using conventional pyridine-catalyzed acetylation; 105minute reaction st 115' C.
Table 111.
Acetylation of Other Hydroxy Compounds
(Average of replicate determinations) Hydroxyl So. Hydroxyl NO. No. of Found by S o . of Found by Pyridine Material Detns. Proposed Method Detns. Catalyzed Acetylation" 240 2 Span 60b 5 237 76 78 Span 65* 4 103 Tween 20b 5 104 98 Tween 40b 5 98 249 Xonylphenol 5 248 245 iltmul 124b 3 245 2 106 Carbowax 1000~ 3 107 428 6 n-Octyl alcohol 20 43 1 30-minute reaction at 115" C.
* Trade names, Atlas Chemical Industries, c
Union Carbide Chemicals Co.
ethers, simple ketones, or aliphatic and aromatic unsaturation as do the methods based on perchloric acid catalyst. LITERATURE CITED
(1) Burns, E. A., Murnca, R. F., ANAL. CHEM.3 1, 397 (1959). (2) Critchfield, F. E., "Indirect AcidBased Methods," Division of Analytical
Inc.
Chemistry, 139th Meeting, ACR, St. Louis, Mo., March 1961. (3) Fritz, J. S., Schenk, G. H., ANAL. CHEM.31, 1808 (1959). (4) . Mehlenbacher, V. C., "Organic Analysis," Vol. I., pp. 1-38, Interscience, New York, 1953. (5) Siggia, S., Hanna, J. G., Culmo, R., ANAL.CHEM.33, 900 (1961). RECEIYED for review August 25, 1961. Accepted Sovember 21, 1961. VOL. 34, NO. 2, FEBRUARY 1962
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